1. Field of the Invention
A gas diffusion electrode can be both an anode or a cathode. If an anode of this type operated on hydrogen is connected against a cathode operated on air or oxygen, a system of that type constitutes a fuel cell. Semi-cells of this technology are suitable as electrodes for the recovery of materials from aqueous solutions. If the fuel gas of the diffusion electrode is oxygen, the electrode is a cathode that can be employed, for example, in the chloralkali electrolysis, recovering energy by reaction of the normally developing hydrogen with oxygen as demonstrated by a decreased cell voltage.
If the fuel gas is hydrogen, the electrode is hydrogen diffusion anode hereinafter referred to as HDA.
2. Description of the Related Art
As shown in the referenced U.S. Pat. Nos. 4,412,894; 4,614,575; and 3,793,165, incorporated herein by reference, basically, the construction of an HDA is as follows: Hydrogen is supplied to a catalyst conductively connected to a metal structure, with solid-liquid-gas hydrogen ions developing on the three-phase limit and enabling current to flow in the event of the presence of a cathode, with voltage applied. This reaction can be used, for example, to operate a zinc-metal recovery electrolysis, with employment of an HDA resulting in a voltage decrease of 1.5 to 1.8 V as compared with a conventional zinc recovery electrolysis meaning 50% savings in electric energy.
This voltage gain, on the one hand, results from the formation enthalpy of the oxy-hydrogen reaction (theoretically, 1.23 V) and the elimination of the overvoltage of the oxygen normally developed on the anode, on the other hand.
The technological application of this reaction known for a long time from the fuel cell technology to the metal recovery electrolysis is faced with a multiplicity of practical difficulties. Conventionally built cells are relatively sensitive vis-a-vis hydrogen gas pressure fluctuations. Though possibly required, it is extremely problematic to repair the plastic membrane, i.e. the separating member between gas chamber and catalyst on the one hand and electrolyte, on the other. The conductive cementation between catalyst carrier and anode metal construction which is still required, constitutes a weak point of the process.